1. Field of the Invention
The present invention is generally in the field of electronic circuits and systems. More specifically, the present invention is in the field of communications circuits and systems.
2. Background Art
Transceivers are typically used in communications systems to support transmission and reception of communications signals through a common antenna, for example at radio frequency (RF) in a cellular telephone or other mobile communication device. A transmitter routinely implemented in such a transceiver in the conventional art may utilize several processing stages to condition and preamplify a transmit signal prior to passing the transmit signal to a power amplifier (PA). For example, the transmit signal may originate as a digital signal generated by a digital block of the transmitter. That digital signal is then typically converted into an analog baseband signal, by means of a digital-to-analog converter (DAC) for instance. The analog baseband signal may then be filtered using a low-pass filter (LPF) and up-converted to RF by a mixer, which is usually implemented as an active circuit. Subsequently, the up-converted signal can be processed by a PA driver, which then passes the preamplified transmit signal to the PA for final amplification and transmission from the transceiver antenna
In a conventional transmitter preamplification chain, pre-amplification gain control may be approximately evenly distributed between lower frequency gain control stages implemented prior to, or in combination with up-conversion, and higher frequency gain control stages after up-conversion. In that conventional design approach, the DAC, LPF, and mixer circuits may collectively contribute a significant portion of the overall gain control, such as approximately fifty percent of the preamplification gain control, for example. Unfortunately, this conventional approach to providing preamplification gain control is fraught with significant disadvantages, owing in part to the substantial inefficiencies resulting from the time and iterative testing required to coordinate calibration amongst the various lower frequency and higher frequency gain control stages.
Despite the disadvantages associated with conventional approaches, those approaches remain in widespread use due to the challenges posed by achieving adequate transmitter pre-amplification gain control at higher frequency using the PA driver stages alone. In order to overcome those challenges, it is desirable to obtain some gain control contribution from each stage of the PA driver, including its output stage to the PA. Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing a variable gain control transformer for implementation as an output transformer of an RF transmitter PA driver and suitable for use in a more modern mobile communication device.